1. Field of the Invention
This invention relates to inclusion-free, uniform, semi-insulating gallium nitride (Al, Ga, In)N substrates, and methods for making the same. The semi-insulating GaN substrate of the invention is useful for the manufacture of electronic devices.
2. Description of the Related Art
Group III-V nitride compounds such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN, and AlGaInN, are direct bandgap semiconductors with a bandgap energy ranging from about 0.6 eV for InN to about 6.2 eV for MN. These materials can be used to produce light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs). The material properties of the III-V nitride compounds are also suitable for fabrication of electronic devices that can be operated at higher temperatures, higher powers, and higher frequencies than conventional devices based on silicon (Si) or gallium arsenide (GaAs).
Most of the III-V nitride devices are grown on foreign substrates such as sapphire (Al2O3) and silicon carbide (SiC) because of the lack of available low-cost, high-quality, large-area native substrates such as GaN substrates. Blue LEDs are mostly grown on insulating sapphire substrates or conductive silicon carbide substrates. Because of the large mismatch in both the thermal expansion coefficients and the lattice constants of the foreign substrate and the GaN film, there are problems associated with the foreign substrates, such as a high defect density, that lead to short device lifetime and bowing of GaN/heteroepitaxial substrate structures. Bowing leads to difficulty in fabricating devices with small feature sizes.
Conductive GaN substrates have recently become available (for example, the conductive GaN substrates that are commercially available from Sumitomo Electric, Inc.). Such conductive GaN substrates are advantageously employed in the manufacturing of blue and UV lasers. However, in a number of electronic applications such as high electron mobility transistors (HEMTs), an insulating or semi-insulating GaN substrate is highly desirable.
Unintentionally doped GaN exhibits n-type conductivity due to the presence of residual n-type impurities as well as crystal defects. Since GaN has a high bandgap energy, a pure and defect-free GaN material should exhibit insulating or semi-insulating electric properties. However, current GaN crystal growth techniques still allow the unintentional incorporation of impurities and various crystal defects such as vacancies and dislocations, which render the GaN crystals conductive.
It is known in the prior art that by introducing deep-level compensating impurities in the crystal, a wide bandgap semiconductor can be made semi-insulating. For example, U.S. Pat. No. 5,611,955 issued to Barrett et al. discloses the use of vanadium doping in silicon carbide to produce a semi-insulating SiC crystal. Similarly, Beccard et al. disclose the use of iron chloride formed by reacting elemental iron with gaseous hydrochloric acid in a vapor phase reactor during the HVPE growth of indium phosphide (InP) to produce iron-doped semi-insulating InP crystals (R. Beccard et al., J. Cryst. Growth, Vol. 121, page 373-380, 1992). The compensating impurities act as deep-level acceptors to trap the otherwise free electrons generated by unintentionally doped n-type impurities and crystal defects. The concentration of the deep-level acceptor is typically higher than the concentration of the free electrons generated by the n-type impurities and crystal defects.
Several deep-level acceptors generated by compensating impurities in gallium nitride (GaN) have been identified in the prior art. For example, Group II metals such as Be, Mg, and Zn, and transition metals such as Fe and Mn, can be incorporated in the GaN crystal resulting in semi-insulating electric properties. The energy level of iron in gallium nitride is well-documented and iron incorporation can result in gallium nitride exhibiting the semi-insulating electric property (see, for example, R. Heitz et al., Physical Review B, Vol. 55, page 4382, 1977). Iron-doped gallium nitride thin films can be grown with metal-organic chemical vapor deposition, molecular beam epitaxy, and hydride vapor phase epitaxy (see, for example, J. Baur et al., Applied Physics Letters, Vol. 64, page 857, 1994; S. Heikman, Applied Physics Letters, Vol. 81, page 439, 2002; and A. Corrion, et al., Journal of Crystal Growth, Vol. 289, page 587, 2006). Zinc-doped gallium nitride thin films grown by hydride vapor phase epitaxy can be semi-insulating as well (N. I. Kuznetsov et al., Applied Physics Letters, Vol. 75, page 3138, 1999).
U.S. Pat. No. 6,273,948 issued to Porowski et al. discloses a method of making a highly resistive GaN bulk crystal. The GaN crystal is grown from molten gallium under an atmosphere of high-pressure nitrogen (0.5-2.0 GPa) and at high temperature (1300-1700° C.). When pure gallium is used, the GaN crystal grown is conductive due to residual n-type impurities and crystal defects. When a mixture of gallium and a Group II metal such as beryllium, magnesium, calcium, zinc, or cadmium is used, the grown GaN crystal is highly resistive, with a resistivity of 104-108 ohm-cm. However, the crystals obtained from molten gallium under the high-pressure, high-temperature process were quite small, on the order of one centimeter, which is not suitable for most commercial electronic applications.
U.S. Patent Application Publication No. 2005/0009310 by Vaudo et al. discloses a large-area semi-insulating GaN substrate grown by hydride vapor phase epitaxy (HVPE). Typically, undoped HVPE-grown GaN is of n-type conductivity due to the residual impurities and crystal defects. By introducing a deep-level doping species during the growth process and at a sufficiently high concentration of the dopant species in the GaN crystal, the grown GaN crystal becomes semi-insulating. Typical dopant species are transitional metals such as iron.
However, during the HVPE growth of single-crystal GaN, there are various surface morphologies observed and these different growth morphologies have different levels of impurity incorporation. U.S. Pat. No. 6,468,347 by Motoki et al. discloses that in the growth of GaN on c-plane substrate by HVPE, the growth surface has inverse pyramidal pits. Because of the presence of the pits on the growing GaN surface, the actual GaN growth takes place both on the non-pitted area, which is normal c-plane growth, and on the faces of the pits, which is non-c-plane growth. U.S. Pat. No. 6,773,504 and No. 7,012,318 by Motoki et al. disclose that GaN growth on the surfaces other than the c-plane has much higher oxygen incorporation.
The presence of inverse pyramidal pits on the GaN crystal surface during HVPE growth results in a non-uniform distribution of n-type impurity concentration in the GaN crystal due to higher impurity incorporation on the non-c-plane surfaces. The impurity concentrations in these pitted areas can be an order of magnitude or more higher than in non-pitted areas. Even when compensating deep-level impurities such as iron are introduced during the crystal growth, the electric characteristics of the grown GaN crystal are not uniform when pits are present during the growth, and GaN wafers made from such crystals will have a non-uniform sheet resistance across the wafer surface. When the as-grown GaN is polished to remove the pits and to produce a smooth surface, the impurity concentration on the surface is still not uniform. The areas where pits were present have a higher oxygen impurity concentration, appear to be darker in color than the surrounding area, and are considered as “inclusions” of more conductive spots. Electronic devices grown on substrates with non-uniform electric properties have lower performance, resulting in lower device yield. Substrates that are “inclusion-free,” or those substrates without non-uniform areas of more conductive spots, would have a more uniform sheet resistance across the wafer surface and higher performance, resulting in higher device yield.
Therefore, there is a compelling need in the art for large-area, inclusion-free, uniform semi-insulating GaN substrates and methods for making such substrates.